package com.connor.basic.thread;

import java.util.AbstractSequentialList;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.Deque;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
import java.util.NoSuchElementException;
import java.util.Queue;


/**
 * 测试为什么需要静态Node
 * @author connor_zeng
 *
 * @param <E>
 */
public class TestLinkedList<E> extends AbstractSequentialList<E> implements
		List<E>, Deque<E>, Cloneable, java.io.Serializable {
	transient int size = 0;

	/**
	 * Pointer to first node. Invariant: (first == null && last == null) ||
	 * (first.prev == null && first.item != null)
	 */
	transient Node<E> first;

	/**
	 * Pointer to last node. Invariant: (first == null && last == null) ||
	 * (last.next == null && last.item != null)
	 */
	transient Node<E> last;

	/**
	 * Constructs an empty list.
	 */
	public TestLinkedList() {
	}

	/**
	 * Constructs a list containing the elements of the specified collection, in
	 * the order they are returned by the collection's iterator.
	 *
	 * @param c
	 *            the collection whose elements are to be placed into this list
	 * @throws NullPointerException
	 *             if the specified collection is null
	 */
	public TestLinkedList(Collection<? extends E> c) {
		this();
		addAll(c);
	}

	/**
	 * Links e as first element.
	 */
	private void linkFirst(E e) {
		final Node<E> f = first;
		final Node<E> newNode = new Node<>(null, e, f);
		first = newNode;
		if (f == null)
			last = newNode;
		else
			f.prev = newNode;
		size++;
		modCount++;
	}

	/**
	 * Links e as last element.
	 */
	void linkLast(E e) {
		final Node<E> l = last;
		final Node<E> newNode = new Node<>(l, e, null);
		last = newNode;
		if (l == null)
			first = newNode;
		else
			l.next = newNode;
		size++;
		modCount++;
	}

	/**
	 * Inserts element e before non-null Node succ.
	 */
	void linkBefore(E e, Node<E> succ) {
		// assert succ != null;
		final Node<E> pred = succ.prev;
		final Node<E> newNode = new Node<>(pred, e, succ);
		succ.prev = newNode;
		if (pred == null)
			first = newNode;
		else
			pred.next = newNode;
		size++;
		modCount++;
	}

	/**
	 * Unlinks non-null first node f.
	 */
	private E unlinkFirst(Node<E> f) {
		// assert f == first && f != null;
		final E element = f.item;
		final Node<E> next = f.next;
		f.item = null;
		f.next = null; // help GC
		first = next;
		if (next == null)
			last = null;
		else
			next.prev = null;
		size--;
		modCount++;
		return element;
	}

	/**
	 * Unlinks non-null last node l.
	 */
	private E unlinkLast(Node<E> l) {
		// assert l == last && l != null;
		final E element = l.item;
		final Node<E> prev = l.prev;
		l.item = null;
		l.prev = null; // help GC
		last = prev;
		if (prev == null)
			first = null;
		else
			prev.next = null;
		size--;
		modCount++;
		return element;
	}

	/**
	 * Unlinks non-null node x.
	 */
	E unlink(Node<E> x) {
		// assert x != null;
		final E element = x.item;
		final Node<E> next = x.next;
		final Node<E> prev = x.prev;

		if (prev == null) {
			first = next;
		} else {
			prev.next = next;
			x.prev = null;
		}

		if (next == null) {
			last = prev;
		} else {
			next.prev = prev;
			x.next = null;
		}

		x.item = null;
		size--;
		modCount++;
		return element;
	}

	/**
	 * Returns the first element in this list.
	 *
	 * @return the first element in this list
	 * @throws NoSuchElementException
	 *             if this list is empty
	 */
	public E getFirst() {
		final Node<E> f = first;
		if (f == null)
			throw new NoSuchElementException();
		return f.item;
	}

	/**
	 * Returns the last element in this list.
	 *
	 * @return the last element in this list
	 * @throws NoSuchElementException
	 *             if this list is empty
	 */
	public E getLast() {
		final Node<E> l = last;
		if (l == null)
			throw new NoSuchElementException();
		return l.item;
	}

	/**
	 * Removes and returns the first element from this list.
	 *
	 * @return the first element from this list
	 * @throws NoSuchElementException
	 *             if this list is empty
	 */
	public E removeFirst() {
		final Node<E> f = first;
		if (f == null)
			throw new NoSuchElementException();
		return unlinkFirst(f);
	}

	/**
	 * Removes and returns the last element from this list.
	 *
	 * @return the last element from this list
	 * @throws NoSuchElementException
	 *             if this list is empty
	 */
	public E removeLast() {
		final Node<E> l = last;
		if (l == null)
			throw new NoSuchElementException();
		return unlinkLast(l);
	}

	/**
	 * Inserts the specified element at the beginning of this list.
	 *
	 * @param e
	 *            the element to add
	 */
	public void addFirst(E e) {
		linkFirst(e);
	}

	/**
	 * Appends the specified element to the end of this list.
	 *
	 * <p>
	 * This method is equivalent to {@link #add}.
	 *
	 * @param e
	 *            the element to add
	 */
	public void addLast(E e) {
		linkLast(e);
	}

	/**
	 * Returns {@code true} if this list contains the specified element. More
	 * formally, returns {@code true} if and only if this list contains at least
	 * one element {@code e} such that
	 * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
	 *
	 * @param o
	 *            element whose presence in this list is to be tested
	 * @return {@code true} if this list contains the specified element
	 */
	public boolean contains(Object o) {
		return indexOf(o) != -1;
	}

	/**
	 * Returns the number of elements in this list.
	 *
	 * @return the number of elements in this list
	 */
	public int size() {
		return size;
	}

	/**
	 * Appends the specified element to the end of this list.
	 *
	 * <p>
	 * This method is equivalent to {@link #addLast}.
	 *
	 * @param e
	 *            element to be appended to this list
	 * @return {@code true} (as specified by {@link Collection#add})
	 */
	public boolean add(E e) {
		linkLast(e);
		return true;
	}

	/**
	 * Removes the first occurrence of the specified element from this list, if
	 * it is present. If this list does not contain the element, it is
	 * unchanged. More formally, removes the element with the lowest index
	 * {@code i} such that
	 * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
	 * (if such an element exists). Returns {@code true} if this list contained
	 * the specified element (or equivalently, if this list changed as a result
	 * of the call).
	 *
	 * @param o
	 *            element to be removed from this list, if present
	 * @return {@code true} if this list contained the specified element
	 */
	public boolean remove(Object o) {
		if (o == null) {
			for (Node<E> x = first; x != null; x = x.next) {
				if (x.item == null) {
					unlink(x);
					return true;
				}
			}
		} else {
			for (Node<E> x = first; x != null; x = x.next) {
				if (o.equals(x.item)) {
					unlink(x);
					return true;
				}
			}
		}
		return false;
	}

	/**
	 * Appends all of the elements in the specified collection to the end of
	 * this list, in the order that they are returned by the specified
	 * collection's iterator. The behavior of this operation is undefined if the
	 * specified collection is modified while the operation is in progress.
	 * (Note that this will occur if the specified collection is this list, and
	 * it's nonempty.)
	 *
	 * @param c
	 *            collection containing elements to be added to this list
	 * @return {@code true} if this list changed as a result of the call
	 * @throws NullPointerException
	 *             if the specified collection is null
	 */
	public boolean addAll(Collection<? extends E> c) {
		return addAll(size, c);
	}

	/**
	 * Inserts all of the elements in the specified collection into this list,
	 * starting at the specified position. Shifts the element currently at that
	 * position (if any) and any subsequent elements to the right (increases
	 * their indices). The new elements will appear in the list in the order
	 * that they are returned by the specified collection's iterator.
	 *
	 * @param index
	 *            index at which to insert the first element from the specified
	 *            collection
	 * @param c
	 *            collection containing elements to be added to this list
	 * @return {@code true} if this list changed as a result of the call
	 * @throws IndexOutOfBoundsException
	 *             {@inheritDoc}
	 * @throws NullPointerException
	 *             if the specified collection is null
	 */
	public boolean addAll(int index, Collection<? extends E> c) {
		checkPositionIndex(index);

		Object[] a = c.toArray();
		int numNew = a.length;
		if (numNew == 0)
			return false;

		Node<E> pred, succ;
		if (index == size) {
			succ = null;
			pred = last;
		} else {
			succ = node(index);
			pred = succ.prev;
		}

		for (Object o : a) {
			@SuppressWarnings("unchecked")
			E e = (E) o;
			Node<E> newNode = new Node<>(pred, e, null);
			if (pred == null)
				first = newNode;
			else
				pred.next = newNode;
			pred = newNode;
		}

		if (succ == null) {
			last = pred;
		} else {
			pred.next = succ;
			succ.prev = pred;
		}

		size += numNew;
		modCount++;
		return true;
	}

	/**
	 * Removes all of the elements from this list. The list will be empty after
	 * this call returns.
	 */
	public void clear() {
		// Clearing all of the links between nodes is "unnecessary", but:
		// - helps a generational GC if the discarded nodes inhabit
		// more than one generation
		// - is sure to free memory even if there is a reachable Iterator
		for (Node<E> x = first; x != null;) {
			Node<E> next = x.next;
			x.item = null;
			x.next = null;
			x.prev = null;
			x = next;
		}
		first = last = null;
		size = 0;
		modCount++;
	}

	// Positional Access Operations

	/**
	 * Returns the element at the specified position in this list.
	 *
	 * @param index
	 *            index of the element to return
	 * @return the element at the specified position in this list
	 * @throws IndexOutOfBoundsException
	 *             {@inheritDoc}
	 */
	public E get(int index) {
		checkElementIndex(index);
		return node(index).item;
	}

	/**
	 * Replaces the element at the specified position in this list with the
	 * specified element.
	 *
	 * @param index
	 *            index of the element to replace
	 * @param element
	 *            element to be stored at the specified position
	 * @return the element previously at the specified position
	 * @throws IndexOutOfBoundsException
	 *             {@inheritDoc}
	 */
	public E set(int index, E element) {
		checkElementIndex(index);
		Node<E> x = node(index);
		E oldVal = x.item;
		x.item = element;
		return oldVal;
	}

	/**
	 * Inserts the specified element at the specified position in this list.
	 * Shifts the element currently at that position (if any) and any subsequent
	 * elements to the right (adds one to their indices).
	 *
	 * @param index
	 *            index at which the specified element is to be inserted
	 * @param element
	 *            element to be inserted
	 * @throws IndexOutOfBoundsException
	 *             {@inheritDoc}
	 */
	public void add(int index, E element) {
		checkPositionIndex(index);

		if (index == size)
			linkLast(element);
		else
			linkBefore(element, node(index));
	}

	/**
	 * Removes the element at the specified position in this list. Shifts any
	 * subsequent elements to the left (subtracts one from their indices).
	 * Returns the element that was removed from the list.
	 *
	 * @param index
	 *            the index of the element to be removed
	 * @return the element previously at the specified position
	 * @throws IndexOutOfBoundsException
	 *             {@inheritDoc}
	 */
	public E remove(int index) {
		checkElementIndex(index);
		return unlink(node(index));
	}

	/**
	 * Tells if the argument is the index of an existing element.
	 */
	private boolean isElementIndex(int index) {
		return index >= 0 && index < size;
	}

	/**
	 * Tells if the argument is the index of a valid position for an iterator or
	 * an add operation.
	 */
	private boolean isPositionIndex(int index) {
		return index >= 0 && index <= size;
	}

	/**
	 * Constructs an IndexOutOfBoundsException detail message. Of the many
	 * possible refactorings of the error handling code, this "outlining"
	 * performs best with both server and client VMs.
	 */
	private String outOfBoundsMsg(int index) {
		return "Index: " + index + ", Size: " + size;
	}

	private void checkElementIndex(int index) {
		if (!isElementIndex(index))
			throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
	}

	private void checkPositionIndex(int index) {
		if (!isPositionIndex(index))
			throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
	}

	/**
	 * Returns the (non-null) Node at the specified element index.
	 */
	Node<E> node(int index) {
		// assert isElementIndex(index);

		if (index < (size >> 1)) {
			Node<E> x = first;
			for (int i = 0; i < index; i++)
				x = x.next;
			return x;
		} else {
			Node<E> x = last;
			for (int i = size - 1; i > index; i--)
				x = x.prev;
			return x;
		}
	}

	// Search Operations

	/**
	 * Returns the index of the first occurrence of the specified element in
	 * this list, or -1 if this list does not contain the element. More
	 * formally, returns the lowest index {@code i} such that
	 * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
	 * or -1 if there is no such index.
	 *
	 * @param o
	 *            element to search for
	 * @return the index of the first occurrence of the specified element in
	 *         this list, or -1 if this list does not contain the element
	 */
	public int indexOf(Object o) {
		int index = 0;
		if (o == null) {
			for (Node<E> x = first; x != null; x = x.next) {
				if (x.item == null)
					return index;
				index++;
			}
		} else {
			for (Node<E> x = first; x != null; x = x.next) {
				if (o.equals(x.item))
					return index;
				index++;
			}
		}
		return -1;
	}

	/**
	 * Returns the index of the last occurrence of the specified element in this
	 * list, or -1 if this list does not contain the element. More formally,
	 * returns the highest index {@code i} such that
	 * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
	 * or -1 if there is no such index.
	 *
	 * @param o
	 *            element to search for
	 * @return the index of the last occurrence of the specified element in this
	 *         list, or -1 if this list does not contain the element
	 */
	public int lastIndexOf(Object o) {
		int index = size;
		if (o == null) {
			for (Node<E> x = last; x != null; x = x.prev) {
				index--;
				if (x.item == null)
					return index;
			}
		} else {
			for (Node<E> x = last; x != null; x = x.prev) {
				index--;
				if (o.equals(x.item))
					return index;
			}
		}
		return -1;
	}

	// Queue operations.

	/**
	 * Retrieves, but does not remove, the head (first element) of this list.
	 *
	 * @return the head of this list, or {@code null} if this list is empty
	 * @since 1.5
	 */
	public E peek() {
		final Node<E> f = first;
		return (f == null) ? null : f.item;
	}

	/**
	 * Retrieves, but does not remove, the head (first element) of this list.
	 *
	 * @return the head of this list
	 * @throws NoSuchElementException
	 *             if this list is empty
	 * @since 1.5
	 */
	public E element() {
		return getFirst();
	}

	/**
	 * Retrieves and removes the head (first element) of this list.
	 *
	 * @return the head of this list, or {@code null} if this list is empty
	 * @since 1.5
	 */
	public E poll() {
		final Node<E> f = first;
		return (f == null) ? null : unlinkFirst(f);
	}

	/**
	 * Retrieves and removes the head (first element) of this list.
	 *
	 * @return the head of this list
	 * @throws NoSuchElementException
	 *             if this list is empty
	 * @since 1.5
	 */
	public E remove() {
		return removeFirst();
	}

	/**
	 * Adds the specified element as the tail (last element) of this list.
	 *
	 * @param e
	 *            the element to add
	 * @return {@code true} (as specified by {@link Queue#offer})
	 * @since 1.5
	 */
	public boolean offer(E e) {
		return add(e);
	}

	// Deque operations
	/**
	 * Inserts the specified element at the front of this list.
	 *
	 * @param e
	 *            the element to insert
	 * @return {@code true} (as specified by {@link Deque#offerFirst})
	 * @since 1.6
	 */
	public boolean offerFirst(E e) {
		addFirst(e);
		return true;
	}

	/**
	 * Inserts the specified element at the end of this list.
	 *
	 * @param e
	 *            the element to insert
	 * @return {@code true} (as specified by {@link Deque#offerLast})
	 * @since 1.6
	 */
	public boolean offerLast(E e) {
		addLast(e);
		return true;
	}

	/**
	 * Retrieves, but does not remove, the first element of this list, or
	 * returns {@code null} if this list is empty.
	 *
	 * @return the first element of this list, or {@code null} if this list is
	 *         empty
	 * @since 1.6
	 */
	public E peekFirst() {
		final Node<E> f = first;
		return (f == null) ? null : f.item;
	}

	/**
	 * Retrieves, but does not remove, the last element of this list, or returns
	 * {@code null} if this list is empty.
	 *
	 * @return the last element of this list, or {@code null} if this list is
	 *         empty
	 * @since 1.6
	 */
	public E peekLast() {
		final Node<E> l = last;
		return (l == null) ? null : l.item;
	}

	/**
	 * Retrieves and removes the first element of this list, or returns
	 * {@code null} if this list is empty.
	 *
	 * @return the first element of this list, or {@code null} if this list is
	 *         empty
	 * @since 1.6
	 */
	public E pollFirst() {
		final Node<E> f = first;
		return (f == null) ? null : unlinkFirst(f);
	}

	/**
	 * Retrieves and removes the last element of this list, or returns
	 * {@code null} if this list is empty.
	 *
	 * @return the last element of this list, or {@code null} if this list is
	 *         empty
	 * @since 1.6
	 */
	public E pollLast() {
		final Node<E> l = last;
		return (l == null) ? null : unlinkLast(l);
	}

	/**
	 * Pushes an element onto the stack represented by this list. In other
	 * words, inserts the element at the front of this list.
	 *
	 * <p>
	 * This method is equivalent to {@link #addFirst}.
	 *
	 * @param e
	 *            the element to push
	 * @since 1.6
	 */
	public void push(E e) {
		addFirst(e);
	}

	/**
	 * Pops an element from the stack represented by this list. In other words,
	 * removes and returns the first element of this list.
	 *
	 * <p>
	 * This method is equivalent to {@link #removeFirst()}.
	 *
	 * @return the element at the front of this list (which is the top of the
	 *         stack represented by this list)
	 * @throws NoSuchElementException
	 *             if this list is empty
	 * @since 1.6
	 */
	public E pop() {
		return removeFirst();
	}

	/**
	 * Removes the first occurrence of the specified element in this list (when
	 * traversing the list from head to tail). If the list does not contain the
	 * element, it is unchanged.
	 *
	 * @param o
	 *            element to be removed from this list, if present
	 * @return {@code true} if the list contained the specified element
	 * @since 1.6
	 */
	public boolean removeFirstOccurrence(Object o) {
		return remove(o);
	}

	/**
	 * Removes the last occurrence of the specified element in this list (when
	 * traversing the list from head to tail). If the list does not contain the
	 * element, it is unchanged.
	 *
	 * @param o
	 *            element to be removed from this list, if present
	 * @return {@code true} if the list contained the specified element
	 * @since 1.6
	 */
	public boolean removeLastOccurrence(Object o) {
		if (o == null) {
			for (Node<E> x = last; x != null; x = x.prev) {
				if (x.item == null) {
					unlink(x);
					return true;
				}
			}
		} else {
			for (Node<E> x = last; x != null; x = x.prev) {
				if (o.equals(x.item)) {
					unlink(x);
					return true;
				}
			}
		}
		return false;
	}

	/**
	 * Returns a list-iterator of the elements in this list (in proper
	 * sequence), starting at the specified position in the list. Obeys the
	 * general contract of {@code List.listIterator(int)}.
	 * <p>
	 *
	 * The list-iterator is <i>fail-fast</i>: if the list is structurally
	 * modified at any time after the Iterator is created, in any way except
	 * through the list-iterator's own {@code remove} or {@code add} methods,
	 * the list-iterator will throw a {@code ConcurrentModificationException}.
	 * Thus, in the face of concurrent modification, the iterator fails quickly
	 * and cleanly, rather than risking arbitrary, non-deterministic behavior at
	 * an undetermined time in the future.
	 *
	 * @param index
	 *            index of the first element to be returned from the
	 *            list-iterator (by a call to {@code next})
	 * @return a ListIterator of the elements in this list (in proper sequence),
	 *         starting at the specified position in the list
	 * @throws IndexOutOfBoundsException
	 *             {@inheritDoc}
	 * @see List#listIterator(int)
	 */
	public ListIterator<E> listIterator(int index) {
		checkPositionIndex(index);
		return new ListItr(index);
	}

	private class ListItr implements ListIterator<E> {
		private Node<E> lastReturned = null;
		private Node<E> next;
		private int nextIndex;
		private int expectedModCount = modCount;

		ListItr(int index) {
			// assert isPositionIndex(index);
			next = (index == size) ? null : node(index);
			nextIndex = index;
		}

		public boolean hasNext() {
			return nextIndex < size;
		}

		public E next() {
			checkForComodification();
			if (!hasNext())
				throw new NoSuchElementException();

			lastReturned = next;
			next = next.next;
			nextIndex++;
			return lastReturned.item;
		}

		public boolean hasPrevious() {
			return nextIndex > 0;
		}

		public E previous() {
			checkForComodification();
			if (!hasPrevious())
				throw new NoSuchElementException();

			lastReturned = next = (next == null) ? last : next.prev;
			nextIndex--;
			return lastReturned.item;
		}

		public int nextIndex() {
			return nextIndex;
		}

		public int previousIndex() {
			return nextIndex - 1;
		}

		public void remove() {
			checkForComodification();
			if (lastReturned == null)
				throw new IllegalStateException();

			Node<E> lastNext = lastReturned.next;
			unlink(lastReturned);
			if (next == lastReturned)
				next = lastNext;
			else
				nextIndex--;
			lastReturned = null;
			expectedModCount++;
		}

		public void set(E e) {
			if (lastReturned == null)
				throw new IllegalStateException();
			checkForComodification();
			lastReturned.item = e;
		}

		public void add(E e) {
			checkForComodification();
			lastReturned = null;
			if (next == null)
				linkLast(e);
			else
				linkBefore(e, next);
			nextIndex++;
			expectedModCount++;
		}

		final void checkForComodification() {
			if (modCount != expectedModCount)
				throw new ConcurrentModificationException();
		}
	}

	private class Node<E> {
		E item;
		Node<E> next;
		Node<E> prev;

		Node(Node<E> prev, E element, Node<E> next) {
			this.item = element;
			this.next = next;
			this.prev = prev;
		}
	}

	/**
	 * @since 1.6
	 */
	public Iterator<E> descendingIterator() {
		return new DescendingIterator();
	}

	/**
	 * Adapter to provide descending iterators via ListItr.previous
	 */
	private class DescendingIterator implements Iterator<E> {
		private final ListItr itr = new ListItr(size());

		public boolean hasNext() {
			return itr.hasPrevious();
		}

		public E next() {
			return itr.previous();
		}

		public void remove() {
			itr.remove();
		}
	}

	@SuppressWarnings("unchecked")
	private TestLinkedList<E> superClone() {
		try {
			return (TestLinkedList<E>) super.clone();
		} catch (CloneNotSupportedException e) {
			throw new InternalError();
		}
	}

	/**
	 * Returns a shallow copy of this {@code LinkedList}. (The elements
	 * themselves are not cloned.)
	 *
	 * @return a shallow copy of this {@code LinkedList} instance
	 */
	public Object clone() {
		TestLinkedList<E> clone = superClone();

		// Put clone into "virgin" state
		clone.first = clone.last = null;
		clone.size = 0;
		clone.modCount = 0;

		// Initialize clone with our elements
		for (Node<E> x = first; x != null; x = x.next)
			clone.add(x.item);

		return clone;
	}

	/**
	 * Returns an array containing all of the elements in this list in proper
	 * sequence (from first to last element).
	 *
	 * <p>
	 * The returned array will be "safe" in that no references to it are
	 * maintained by this list. (In other words, this method must allocate a new
	 * array). The caller is thus free to modify the returned array.
	 *
	 * <p>
	 * This method acts as bridge between array-based and collection-based APIs.
	 *
	 * @return an array containing all of the elements in this list in proper
	 *         sequence
	 */
	public Object[] toArray() {
		Object[] result = new Object[size];
		int i = 0;
		for (Node<E> x = first; x != null; x = x.next)
			result[i++] = x.item;
		return result;
	}

	/**
	 * Returns an array containing all of the elements in this list in proper
	 * sequence (from first to last element); the runtime type of the returned
	 * array is that of the specified array. If the list fits in the specified
	 * array, it is returned therein. Otherwise, a new array is allocated with
	 * the runtime type of the specified array and the size of this list.
	 *
	 * <p>
	 * If the list fits in the specified array with room to spare (i.e., the
	 * array has more elements than the list), the element in the array
	 * immediately following the end of the list is set to {@code null}. (This
	 * is useful in determining the length of the list <i>only</i> if the caller
	 * knows that the list does not contain any null elements.)
	 *
	 * <p>
	 * Like the {@link #toArray()} method, this method acts as bridge between
	 * array-based and collection-based APIs. Further, this method allows
	 * precise control over the runtime type of the output array, and may, under
	 * certain circumstances, be used to save allocation costs.
	 *
	 * <p>
	 * Suppose {@code x} is a list known to contain only strings. The following
	 * code can be used to dump the list into a newly allocated array of
	 * {@code String}:
	 *
	 * <pre>
	 * String[] y = x.toArray(new String[0]);
	 * </pre>
	 *
	 * Note that {@code toArray(new Object[0])} is identical in function to
	 * {@code toArray()}.
	 *
	 * @param a
	 *            the array into which the elements of the list are to be
	 *            stored, if it is big enough; otherwise, a new array of the
	 *            same runtime type is allocated for this purpose.
	 * @return an array containing the elements of the list
	 * @throws ArrayStoreException
	 *             if the runtime type of the specified array is not a supertype
	 *             of the runtime type of every element in this list
	 * @throws NullPointerException
	 *             if the specified array is null
	 */
	@SuppressWarnings("unchecked")
	public <T> T[] toArray(T[] a) {
		if (a.length < size)
			a = (T[]) java.lang.reflect.Array.newInstance(a.getClass()
					.getComponentType(), size);
		int i = 0;
		Object[] result = a;
		for (Node<E> x = first; x != null; x = x.next)
			result[i++] = x.item;

		if (a.length > size)
			a[size] = null;

		return a;
	}

	private static final long serialVersionUID = 876323262645176354L;

	/**
	 * Saves the state of this {@code LinkedList} instance to a stream (that is,
	 * serializes it).
	 *
	 * @serialData The size of the list (the number of elements it contains) is
	 *             emitted (int), followed by all of its elements (each an
	 *             Object) in the proper order.
	 */
	private void writeObject(java.io.ObjectOutputStream s)
			throws java.io.IOException {
		// Write out any hidden serialization magic
		s.defaultWriteObject();

		// Write out size
		s.writeInt(size);

		// Write out all elements in the proper order.
		for (Node<E> x = first; x != null; x = x.next)
			s.writeObject(x.item);
	}

	/**
	 * Reconstitutes this {@code LinkedList} instance from a stream (that is,
	 * deserializes it).
	 */
	@SuppressWarnings("unchecked")
	private void readObject(java.io.ObjectInputStream s)
			throws java.io.IOException, ClassNotFoundException {
		// Read in any hidden serialization magic
		s.defaultReadObject();

		// Read in size
		int size = s.readInt();

		// Read in all elements in the proper order.
		for (int i = 0; i < size; i++)
			linkLast((E) s.readObject());
	}
}
